Devise.setup do |config|
...
# ==> Configuration for :saml_authenticatable
# Create user if the user does not exist. (Default is false)
config.saml_create_user = true
# Update the attributes of the user after a successful login. (Default is false)
config.saml_update_user = true
# Set the default user key. The user will be looked up by this key. Make
# sure that the Authentication Response includes the attribute.
config.saml_default_user_key = :email
# Optional. This stores the session index defined by the IDP during login. If provided it will be used as a salt
# for the user's session to facilitate an IDP initiated logout request.
config.saml_session_index_key = :session_index
# You can set this value to use Subject or SAML assertation as info to which email will be compared
# If you don't set it then email will be extracted from SAML assertation attributes
config.saml_use_subject = true
# You can support multiple IdPs by setting this value to a class that implements a #settings method which takes
# an IdP entity id as an argument and returns a hash of idp settings for the corresponding IdP.
config.idp_settings_adapter = nil
# You provide you own method to find the idp_entity_id in a SAML message in the case of multiple IdPs
# by setting this to a custom reader class, or use the default.
# config.idp_entity_id_reader = DeviseSamlAuthenticatable::DefaultIdpEntityIdReader
# You can set a handler object that takes the response for a failed SAML request and the strategy,
# and implements a #handle method. This method can then redirect the user, return error messages, etc.
# config.saml_failed_callback = nil
# Configure with your SAML settings (see [ruby-saml][] for more information).
config.saml_configure do |settings|
settings.assertion_consumer_service_url = "http://localhost:3000/users/saml/auth"
settings.assertion_consumer_service_binding = "urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST"
settings.name_identifier_format = "urn:oasis:names:tc:SAML:2.0:nameid-format:transient"
settings.issuer = "http://localhost:3000/saml/metadata"
settings.authn_context = ""
settings.idp_slo_target_url = "http://localhost/simplesaml/www/saml2/idp/SingleLogoutService.php"
settings.idp_sso_target_url = "http://localhost/simplesaml/www/saml2/idp/SSOService.php"
settings.idp_cert = <<-CERT.chomp
-----BEGIN CERTIFICATE-----
1111111111111111111111111111111111111111111111111111111111111111
1111111111111111111111111111111111111111111111111111111111111111
1111111111111111111111111111111111111111111111111111111111111111
1111111111111111111111111111111111111111111111111111111111111111
1111111111111111111111111111111111111111111111111111111111111111
1111111111111_______IDP_CERTIFICATE________111111111111111111111
1111111111111111111111111111111111111111111111111111111111111111
1111111111111111111111111111111111111111111111111111111111111111
1111111111111111111111111111111111111111111111111111111111111111
1111111111111111111111111111111111111111111111111111111111111111
1111111111111111111111111111111111111111111111111111111111111111
1111111111111111111111111111111111111111111111111111111111111111
1111111111111111111111111111111111111111111111111111111111111111
111111111111111111
-----END CERTIFICATE-----
CERT
end
end

The attribute mappings are very dependent on the way the IdP encodes the attributes.
In this example the attributes are given in URN style.
Other IdPs might provide them as OID's or other means.

You are now ready to test it against an IdP.
When the user goes to /users/saml/sign_in he will be redirected to the login page of the IdP.
Upon successful login the user is redirected to devise user_root_path.

Supporting Multiple IdPs

If you must support multiple Identity Providers you can implement an adapter class with a #settings method that takes an IdP entity id and returns a hash of settings for the corresponding IdP. The config.idp_settings_adapter then must be set to point to your adapter in config/initializers/devise.rb. The implementation of the adapter is up to you. A simple example may look like this:

Detecting the entity ID passed to the settings method is done by config.idp_entity_id_reader.
By default this will find the Issuer in the SAML request.
You can support more use cases by writing your own and implementing the .entity_id method.

Identity Provider

If you don't have an identity provider an you would like to test the authentication against your app there are some options:

Use ruby-saml-idp. You can add your own logic to your IdP, or you can also set it as a dummy IdP that always sends a valid authentication response to your app.

Use an online service that can act as an IdP. Onelogin, Salesforce, Okta and some others provide you with this functionality

Install your own IdP.

There are numerous IdPs that support SAML 2.0, there are propietary (like Microsoft ADFS 2.0 or Ping federate) and there are also open source solutions like Shibboleth and simplesamlphp.

SimpleSAMLphp was my choice for development since it is a production-ready SAML solution, that is also really easy to install, configure and use.

Logout

Logout support is included by immediately terminating the local session and then redirecting to the IdP.

Logout Request

Logout requests from the IDP are supported by the idp_sign_out end point. Directing logout requests to users/saml/idp_sign_out will logout the respective user by invalidating their current sessions.
saml_session_index_key must be configured to support this feature.

Signing and Encrypting Authentication Requests

ruby-saml 1.0.0 supports signature and decrypt. Teh only requirement is to place the public certificate and the private key. Please reffer to these features in the ruby-saml documentation here

Thanks

The continued maintenance of this gem could not have been possible without the hard work of Adam Stegman and Mitch Lindsay. Thank you guys for keeping this project alive.

Thanks to all other contributors that have also helped us make this software better.